When stimulating production zones in wells, a first ball is dropped into the well and flows with the well fluid until it reaches a ball seat which it cannot pass, causing the ball to seat in the ball seat of a first sleeve. A continuous pumping of fluid into the well then results in a pressure on the ball moving the sleeve from a closed position to an open position. As the sleeve opens, the fluid enters the formation surrounding the well, and the stimulation process can begin. A second production zone is stimulated be dropping a second ball which is larger than the first ball, which flows in the fluid until it reaches a ball seat in another sleeve positioned closer to the top of the well than the first sleeve. The second ball seats in the ball seat of the second sleeve, the sleeve is forced open, and the stimulation process of the second production zone can begin. In this way, multiple balls can be dropped to stimulate multiple sections of the well.
When the stimulation of the production zones has ended, an operation tool is submerged into the well to retrieve the ball seated in the sleeve closest to the surface, e.g. by drilling a hole in the ball. The first operation tool is then retracted from the well again, and the operation tool is, in a second run, submerged into the well to retrieve the next ball. The retrieval process is continued until all the balls have been retrieved, and oil production can be initiated by opening all the sleeves again.
Using this ball dropping process is inexpensive, but also very time-consuming since the balls have to be retrieved one by one. Furthermore, retrieving a round ball rolling in a ball seat can be very difficult, and the retrieval process may therefore fail.